1. Field of the Invention
The present invention relates to a process for producing toner and, in particular, to a process for producing a toner which has a high level of staining power, brightness, and saturation, and which excels in charging stability.
2. Description of the Related Art
Generally, when manufacturing toner by grinding, a melt-kneaded intermediate mixture is formed. The melt-kneaded mixture is composed of: a binding resin such as a styrene-type resin, a styrene-acrylic-type resin, an epoxy resin, or a polyester resin; a coloring agent; an offset prevention agent; a charge controlling agent, etc. Afterwards, the melt-kneaded mixture is cooled, ground, and classified into the desired toner particles.
Melt-kneading is generally conducted by using a kneader such as a screw-type extruder, a pressure/heat kneader, a two-roll mill, a three-roll mill, or a Banbury mixer to obtain a melt-kneaded material out of the above melt-kneaded mixture through a single dispersion process. In a color toner for electrophotography, this melt-kneading process is a significant stage in the toner manufacture, since the nature of the dispersion among the toner particles of the coloring agent, the charge controlling agent, etc. depends on the melt-kneading process.
An uneven dispersion of the coloring agent contained in the binding resin results, in the case of color toner, in a reduced level of staining power and saturation and the color tone being rather pale. Furthermore, if the coloring agent undergoes a secondary agglomeration and becomes segregated in the binding resin, an unevenness in density will result.
Moreover, a defective dispersion among the toner particles of the charge control agent may, in some cases, cause a defect in the charging property, which is fatal to any toner for use in electrophotography. For example, there can occur a delay in the triboelectric charging of the toner, resulting in a reduced level of triboelectric charging. In addition, such a defective dispersion will cause the triboelectric charging property of the toner to vary to an excessive degree, e.g., between low-temperature/low-humidity and high-temperature/high-humidity conditions.
If copying is repeated to make a large number of photocopies, the triboelectric charging properties of the toner deteriorates. The toner is then likely to exhibit a broad triboelectric charge distribution, which means it has become rather poor in its environmental stability.
There have been proposed a number of methods for solving the problem of defective dispersion of the coloring agent and/or the charge controlling agent in the toner. According to one of these methods, the melt-kneaded material is passed through a kneading machine a plurality of times in order to improve the dispersion. With this method, however, a degree of dispersibility beyond the inherent dispersion equilibrium of the kneading machine cannot be obtained. Accordingly, it is difficult to obtain the expected result even if the kneaded material is passed through the kneading machine a plurality of times. In another method, a preparatory mixing machine such as a flow mixer, a Henschel mixer, a micro-speed mixer, or a flash mixer, is used. With this method, however, the dispersion is, in most cases, effected by means of mixing blades, which are rotated at high speed, causing the temperature in the container to rise during mixing, and the binding resin to adhere to the blades or to the inner container walls, and/or to melt and stick to the main shaft section.
Still another method is the master batch method, according to which a resin composition containing a coloring agent of high density is prepared beforehand. The resin composition is then diluted by the same binding resin to obtain a desired kneaded material. For example, Japanese Patent Application Laid-Open No. 62-30259 discloses a method according to which a finely ground master batch is melted and diluted. A problem with this method is that the fine-grinding process must be repeated a plurality of times before a desired toner can be obtained, so that there is an enormous consumption of energy and at an unduly high cost. Further, this method gives no consideration to the dispersibility of the charge controlling agent, which greatly influences the triboelectric charging characteristic of the toner.
Japanese Patent Application Laid-Open No. 61-117565 and 61-91666 disclose toner producing processes according to which the binding resin and the coloring agent are dispersed in a solvent, which is then removed, thereby obtaining toner. These methods are improved as compared to a simple melt-kneading method in terms of the evenness in the dispersion of the coloring agent. However, since the granulation is effected in water, the toner particles are liable to contain water, or the particle surfaces are subject to hydration, so that a satisfactory triboelectric charging characteristic is hard to obtain, a condition which is not favorable to electrophotography.
Japanese Patent Application Laid-Open No. 61-117565 and 61-156054 disclose methods according to which the binding resin and the charge controlling agent are previously dissolved and dispersed in a solvent, and, after removing the solvent, the dispersed composition is melt-kneaded together with the binding resin and the coloring agent. The present inventors examined these methods and found them very satisfactory in regard to dispersion, but rather undesirable in terms of triboelectric charging characteristic of toner. For example, when mixing the toner with a carrier, the triboelectric charging ability of toner remained at a low level, and the amount of the charge obtained through temporary friction with the carrier inside the electrophotographic developing apparatus tended to gradually decrease. The present inventors presume this phenomenon is attributable to the fact that the effective amount of charge controlling agent existing on the toner surface decreases due to the excessively good dispersion of the charge controlling agent, preventing the toner and the carrier from being charged to a satisfactory degree. Electrostatically, it is said that charging caused by mutual friction between material surfaces involves at most only that portion of the material to the depth of several thousand Angstroms from material surface. In view of this, it is to be assumed that that portion of the charge controlling agent in the center of the toner particle, or that deeper than several .ANG. from the toner particle surface, has substantially nothing to do with the triboelectric charging.